Motion Sickness and Postural Instability after Prolonged Exposure to an Altered Gravitoinertial Force Environment (+2Gz)

Eric Martin(a) and Gary Riccio(b)

a. Systerns Research Laboratories, Inc., Dayton, OH, U.S.A. b. University of Illinois at Urbana-Champaign, Urbana, IL, U.S.A.

Publication Information: Book Title: Studies in Perception and Action II: Posters Presented at the VIIth International Conference on Event Perception and Action, August 8-13, 1993, University of British Columbia, Vancouver BC, Canada. Contributors: S. Stavros Valenti - editor, John B. Pittenger - editor, International Conference on Event Perception and Action - orgname. Publisher: Lawrence Erlbaum Associates. Place of Publication: Hillsdale, NJ. Publication Year: 1993. Page Number: 366-369.

Space Motion Sickness (SMS) is an operationally significant problem during Space Shuttle Missions. Symptoms of SMS include loss of appetite, malaise, lethargy, retching, vomiting, a strong desire to keep the head still, and decrements in performance. The most severe levels of SMS include multiple incidents of retching or vomiting and performance decrements that last as long as 72 hours. NASA research has found that through 34 missions (STS-1 through STS-34) 67% of Shuttle crew members experienced some degree of SMS during their first mission (Davis, Vanderploeg, Santy, Jennings, & Stewart, 1988; Davis & Beck, 1990). 48% of these astronauts experienced "mild" SMS, 34% experienced "moderate" SMS, and 17% experienced "severe" levels of SMS. Since STS-26 (Challenger) 26 crew members have been involved in subsequent missions. Twelve of these 26 crew members (46%) showed symptoms of SMS. Fifteen crew members reported no change in their symptomology, while nine indicated that their symptoms were less severe. Two reported that symptoms were worse in subsequent missions. Incidence of SMS among "experienced" crew members (those who flew subsequent missions) was not statistically different from the incidence among crew members on their first flight ( Davis & Beck, 1990). Orbital flight is an environment in which the sum of gravitational and inertial forces is zero and, consequently, one in which astronauts are in a state of persistent weightlessness ( Stoffregen & Riccio, 1988). There is general agreement that SMS occurs because of this altered gravitoinertial environment, although the nature of the causal mechanisms involved is a matter of some controversy ( Stoffregen & Riccio, 1991). Orbital flight is not the only way to produce a persistence change in the force environment. Gravitoinertial force can be enhanced by prolonged acceleration such as experienced during curved trajectories of self motion. The centrifugal "force" due to the centripetal acceleration of such a moving reference frame adds, vectorially, to the "force" of gravity. This method of enhancing and controlling gravitoinertial magnitude can be achieved with a human centrifuge. The magnitude of gravitoinertial force is generally scaled to gravity and, thus, is referred to as "1G" (normal terrestrial conditions) or "2G" (twice the magnitude of gravitational force), for example. More specifically, "+2Gz " refers to a

magnitude of force that is twice the magnitude of gravity in the body axis and directed away from the head (using a body-axis reference frame). There is intriguing evidence that prolonged exposure to enhanced Gz and subsequent reintroduction to 1Gz produces symptoms similar to those associated with SMS (e.g., Bles & de Graaf, 1992; Ockels, Furrer, & Messerschmid, 1990). Bles and de Graaf ( 1992) exposed 27 subjects to a +3G z load for 1.0 to 1.5 hours in a centrifuge--centripetal acceleration was in the horizontal plane and the subject's body was supine with the gravitoinertial force vector directed away from the head. From the theoretical perspective of Riccio & Stoffregen (1991), it is noteworthy that Bles and de Graaf evaluated postural stability before and immediately after exposure to +3Gz. Postural stability was disrupted in some conditions after centrifugation, especially during head movement. Head movements also tended to provoke motion sickness. Presumably for this reason "the subjects therefore intuitively prevented head movements: they kept the head fixed to the trunk and walked slowly like a robot" ( Bles & de Graaf, 1992, p. 277). These investigators also noted that the provocativeness of head movements depended on whether movement was in the pitch, roll, or yaw axis and on the direction of gravitoinertial force with respect to the subject's body. "In general terms this means that only those head movements are provocative that change the position of the head relative to the direction of gravity" (p. 278). Such effects suggest that postural instability, itself, is the cause of motion sickness and that adaptation may involve the use of abnormal, yet robust, postural control strategies (Riccio & Stoffregen, 1991). If postural instability is the proximal cause of motion sickness, similarity between different forms of motion sickness should be based more directly on the characteristics of postural instability than on the environmental transformations that lead to instability; although it should be noted that these factors are not completely separable (see Riccio & Stoffregen, 1991). Motion sickness and postural instability after prolonged exposure to enhanced Gz may be similar, in some respects, to SMS and post-flight instability (Ockels et al., 1990). Such effects of enhanced z are also interesting in their own right because variations in the gravitoinertial environment would be experienced by the crew and passengers of hypersonic vehicles such as the National Aerospace Plane (NASP). Methods and results Variations in G were produced with the "Dynamic Environment Simulator" (DES), a human centrifuge with a 5.8 m radius, in the Armstrong Laboratory, WrightPatterson AFB, Ohio. Postural stability was evaluated, in a variety of conditions, with a NeuroCom Equitest System ( Nashner & McCollum, 1985). This report describes the results for anterior-posterior (a-p) postural sway obtained for eleven subjects before and immediately after 90 minutes in the DES at +2G z. Two of these subjects were the only ones in the study that became motion sick as indicated by self-report scores on the Graybiel Scale or through observation by the attending physician. Data on the location of the center or pressure (Cp) at the support surface were sampled with the Equitest System at a rate of 50 Hz over a period of 20 s. Power spectra for a-p sway (variations in Cp) in the eyes-open condition are presented in Figure 1. Power spectra were very similar for all

subjects and conditions for which there was no evidence of motion sickness. Spectral peaks indicating "pathological tremor," however, were obtained only for the two subjects who became motion sick and only when they were motion sick (i.e., after centrifugation, but not before). These results are consistent with the hypothesis of Riccio & Stoffregen (1991) that instability in the postural system could be revealed by pathological tremor. The results are also consistent with the hypothesis that postural instability should be apparent, after exposure to provocative environments, only in subjects who become motion sick (Hamilton, Kantor, & Magee, 1989; Riccio & Stoffregen, 1991).

Figure 1. Power spectra for a-p sway measured as movement in the center of pressure on the support surface. Power spectra for all pre- and post-centrifuge conditions in which no motion sickness was observed are contained within the lightly-shaded monotonic band. This band provides a reference spectrum in both panels. The data for the two motion-sick subjects are represented separately in (a) and (b). The darkly-shaded spectral peaks represent pathological tremor in the post-centrifuge conditions for the two subjects who were motion sick. References

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